Three Phase Transformer And Auto Transformer

Prepared By:RITU K.R.

Assistant Professor (EE)SISTec-E

2

3- Transformer Construction

Left: A 1300 MVA, 24.5/345 kV, 60Hz transformer with forced oiland air (fan) cooling.Right: A 60 MVA, 225/26.4 kV, 60 Hz showing the conservator.

3

3- Transformer Construction

SISTec-e 4

Review of balanced three phase circuits

• Two possible configurations: Star (Y) and delta ()

•Star has neutral, delta does not

5

Star (Y) connection•Line current is same as phase current •Line-Line voltage is 3 phase-neutral voltage

•Power is given by 3 VL-LI Lcos or 3VphIphcos

6

Delta () connection•Line-Line voltage is same as phase voltage•Line current is 3 phase current

•Power is given by 3 VL-LI Lcos or 3VphIphcos

SISTec-e 7

Typical three phase transformer connections

SISTec-e 8

Other possible three phase Transformers Connection • Y- zigzag

•- zigzag

•Open Delta or V

•Scott or T

The Auto Transformer

Sometimes, it is desirable to change the voltage by a small amount (for instance, when the consumer is far away from the generator and it is needed to raise the voltage to compensate for voltage drops).In such situations, it would be expensive to wind a transformer with two windings of approximately equal number of turns. An autotransformer (a transformer with only one winding) is used instead. Diagrams of step-up and step-down autotransformers:

Common winding

Series winding

Series winding

Common winding

Output (up) or input (down) voltage is a sum of voltages across common and series windings.

10

Autotransformer

•Primary and secondary on the same winding. Therefore there is no galvanic isolation.

11

Features of Autotransformer

Lower leakage

Lower losses

Lower magnetizing current

Increase kVA rating

No galvanic Isolation

SISTec-e 12

How are three phase Transformers made

• Either by having three single phase transformers connected as threephase banks.

•Or by having coils mounted on a single core with multiple limbs

•The bank configuration is better from repair perspective, whereas the single three phase unit will cost less ,occupy less space, weighs less andis more efficient

Thank You

13